1. Field of the Invention
The present invention relates to a projection television apparatus, and more particularly to an apparatus which combines television images from three monochromatic cathode-ray tubes into the color image by dichroic mirrors and projects the resulting color image on the screen through a lens.
2. Description of Prior Art
In order to provide a large-picture television apparatus, many types of projection television apparatuses have been proposed. There has been proposed a projection television apparatus as shown in FIG. 1, which illustrates the structure of the main block. The outline of the structure and the principle of this type of apparatus is described below. The main block consists of three monochromatic cathode-ray tubes (1R), (1G) and (1B), two dichroic mirrors (2R) and (2B), and a lens (3). The three tubes (1R), (1G) and (1B) project red, green and blue images, respectively. One dichroic mirror (2R) is a red-type dichroic mirror from which red light is reflected and which green and blue light passes through. The other dichroic mirror (2B) is a blue-type dichroic mirror from which blue light is reflected and which red and green light passes through. The red tube (1R) and the blue tube (1B) are placed on opposite sides of the green tube (1G) in a pattern corresponding to the character "T", with their faceplates (4R), (4G) and (4B) close to each other. In the space shaped like a rectangular parallelepiped formed by the faceplates (4R), (4G) and (4B) of the three tubes, are positioned the two dichroic mirrors (2R) and (2B) which are crossed at line 5 at a right angle to each other when viewed from the edges thereof and with the surfaces of the plates facing the tubes. The angle between a perpendicular to the surface of a dichroic mirror and an optical axis of a tube directed thereat is 45.degree.. The lens (3) is placed on the axis of the green tube (1G), and the red and blue tubes are opposed and coaxial, with the axes of the three tubes meeting at the crossing line 5 of the mirrors so that the dichroic mirrors (2R) and (2B) are surrounded by the tubes (1R), (1G) and (1B), and the lens (3). Employing such apparatus as shown in FIG. 1, the red, green and blue images from the three tubes (1R), (1G) and (1B) are combined into the color image by the dichroic mirrors (2R) and (2B), and the resulting color image is enlarged and projected on a screen (not shown) by the lens (3).
The apparatus shown in FIG. 1 has the following advantages because the optical axis of the red, green and blue light-beams projected from the lens (3) are in accord.
1. The projection distance from the lens (3) to the screen can be varied simply by shifting the lens (3) back and forth.
2. Because each optimum image of red, green and blue can be adjusted to agree with the others on the screen, this apparatus provides a high-resolution image over the entire screen area. 3. Because the shape of the image of each color projected on the screen is similar to the image from the corresponding tube (1R), (1G) and (1B), circuits for convergence adjustment are not necessary for this apparatus in principle. Even if the convergence adjustment is necessary, it is easy and convergence can be fully achieved.
4. Even if using a highly-directional screen, observers never feel color-change due to moving their observation position relative to the screen.
On the other hand, this apparatus shown in FIG. 1 has a weak point as follows. When the tubes (1R), (1G) and (1B) are driven so as to display uniform-brightness images, a phenomenon that the color is dependent on the location in a horizontal or vertical direction on the screen is observed (this is called so-called color shading). The extreme of this phenomenon is observed when using a lens with a small F-number and when using a wide angle lens.
This phenomenon can be illustrated as follows.
FIGS. 2a and 2b show examples of the relation between the transmission coefficient and the wavelength of a ray of light, for the red-type dichroic mirror (2R) and the blue-type dichroic mirror (2B), respectively. The parameter in these figures is the angle of incidence .theta..sub.i. Curves (R), (G) and (B) drawn by dashed lines show the emitting spectrums of the phosphors used on the tubes (1R), (1G) and (1B), respectively. As can be seen from FIGS. 2a and 2b, a cutoff wavelength (a cutoff wavelength is defined here as the wavelength where the transmission coefficient is 50%) depends on the angle of incidence .theta..sub.i. As the angle of incidence .theta..sub.i increases, the cutoff wavelength becomes shorter. The relation between the reflectivity and the wavelength is almost the same as that obtained when the above graph is turned upside-down, and the longitudinal axis represents the the reflectivity (in FIGS. 2a and 2b above).
It is seen from FIGS. 2a and 2b that when the cutoff wavelength shifts, the transmission coefficient or reflectivity of the dichroic mirrors (2R) and (2B) changes at near the cutoff wavelength, so that the light intensity of the transmitted ray or the reflected ray changes. Consider the characteristics of the red-type dichroic mirror (2R), assuming that the cutoff wavelength of the red-type dichroic mirror (2R), for .theta..sub.i =45.degree., is between the red emitting spectrum (R) and the green emitting spectrum (G), as shown in FIG. 2a. For .theta..sub.i =30.degree., the cutoff wavelength gets close to the red emitting spectrum (R), and the light intensity of the red reflected ray becomes weaker than the for .theta..sub.i =45.degree., although the light intensity of the green transmitting ray doesn't change very much. And also for .theta..sub.i =60.degree., the cutoff wavelength gets close to the green emitting spectrum (G), and the light intensity of the green transmitting ray becomes weaker than for .theta..sub.i =45.degree., although the light intensity of the red reflected ray doesn't change very much. The blue-type dichroic mirror (2B) causes a similar phenomenon.
It is also seen from FIGS. 2a and 2b that changes of the angle of incidence .theta..sub.i cause a change of the transmission coefficient at some wavelength, so that the light intensity of the transmitted ray or the reflected ray from the dichroic mirrors (2R) and (2B) is changed.
FIG. 3 shows the brightness distribution of the red, green and blue images on the screen through a line perpendicular to the line of intersection (5) as shown in FIG. 1. The brightness of each color decreases from the center toward the right end and the left end. This is because the light beam through the lens (3) becomes thin as the field angle increases. The curves shown in FIG. 3 are not symmetric with respect to the right side and the left side. This is because the angles of incidence on the dichroic mirror are different on the right side and left side, so that this difference causes the difference of both the cutoff wavelength and the transmission coefficient between the right side and the left side. Comparing the brightness distributions of each color, it is seen that the distributions of red and blue have the same tendency, but the distribution of green has the opposite tendency to that of red and blue, and that the brightness ratios of red, green and blue depend on the location. The brightness ratio is defined as the ratio of brightness of one color in question to total brightness of the three colors. The difference of the brightness ratios becomes greater toward the periphery of the projected image, which causes the color-shading phenomenon.
As a method that alleviates the color shading problem mentioned above, there is a method of displaying images, whose brightness distributions are dependent upon the location in the horizontal or vertical direction, on three tubes (1R), (1G) and (1B), so that the brightness ratios are kept uniform over all areas of the screen. According to this method the color shading can be considerably improved. But this method is undesirable, because overload occurs on the area of the tubes (1R), (1G) and (1B) where high-brightness images are displayed. This overload causes a decrease of the life of the tubes (1R), (1G) and (1B). And also this method is undesirable, since the area of the tubes (1R), (1G) and (1B) where low-brightness images are displayed.